Relieved thread rolling die



l INVENTOR Y EDWARD c. HANNA BY ATTORNEYS United States Patent @thee Patented July 28,` 1959 2,896,484 yRELrEvED 'rjrmnxnRoLLmG nur Edward C. Hanna, Waynesboro, .1?a assigner` to Landis Machine Company, WaynesborlPaga corporation of This invention relates to dies for rolling threads and has particular refeence to an improved formation of the helical` thread forming profile adjacent the edge of the die.

The dies to which the invention is especially adapted are generally cylindrical form` and are provided with a plurality of helical ridges on` their cir'cmferences, the contour ofthe ridges being exactly, complementaryto that of the thread or other helical profile to be rolled on the work piece. Where each of such ridges intersects the plane side surface of the die a partialV ridge profile is formedwhich has the original` contour on one side and a planar surface on the other. It is well known that,` because of the unequal lateral force components. on such partial profiles, they fracture easily in use, leadinglto rapid deterioration of the entire working surface of the die. i v l I While it has been proposed to relieve such partial profiles or thread starts by grinding away a'portion ofthe thread start adjacent the edge ofthe die,A these relieved sectionsfhave either included a relatively widenedV crest, which makes penetration of the work piece difficult, or have included sharp corners inthe flanks of the relieved thread'forming profile.

vIn either case the prior known relieved profiles produce sharp corners along the flanks of the fnishedlthread; As is well known, a sharp indentationlaroundfthecircumference of a metal part reduces the. tensilestrength thereof quite substantially. Thus the `use of: either of these two profiles is precluded inthe manufacture of parts specially designedfor 4high tensile strength. A

In accordance with the present invention the relieving profile of each thread. start on a` rolling die. is formed'in a manner to be describedbelow, to. minimize the. effect of sharp corners in the finished threadnd thus `to greatly enhance the tensile strength of the. product.

' Furthermore, in accordance with. known prior` practices, the length of therelieved. portion of the thread-s limited to sornewhat less than the circumferential` extent between two adjacent thread starts, the chief reason therefor being thepossibility of grinding the relieffprole automatically. It has been found advantageous'in certain cases to in crease the'length' of the relieved. portion in order to aid penetration of the relatively nblunt relieved crest and to insure a proper termination of the threadon the work piece, particula rly `in cas-e offa work piece in which the portion to be threaded is reduced in diameter with respect to the unthreaded portion.` In such cases` thefwidth `of the transition portion between the two diameters determines the axial-extentof the relief on'theA die forbest'results and `such axialextent'should therefore' be' adaptable to the configuration of the work piece.

The relief profile of the invention has further important advantagesin the case of Acme or otherthreads'fhaving a dat original crest instead ofv asharp crest. AIf the flat crest. is permitted to retain its originali` width over the relieved portion of the dieror if it. is widened, the difficult'y ofpenetrating the work piece causesa verystrong bending. moment` to. be. applied thereto, Adeforming the 24 work piece in an undesirable manner. Narrowing and sharpening the crest profile over the radiallyrelievedpor, tionlof the die in accordance with the present invention facilitates workl penetration and minimizes: the` bending moment in addition to the other. advantages enumerated above.

Accordingly it is an object of` the invention` to` provide a die for Vcold-rolling screw threads and the like in` which the thread starts'at'the side thereof `are relieved radially with a contour designed to minimize sharp groovesY in the resultant work piece. A

Another` object` of theA invention is to provide novel thread rolling dies whoseindividual thread ridges are gradually reduced in height toward the edge. of thedie, the length of the portion so relieved being variable over a wide range so as to bev adaptable to work piece dimensions.

It is a further object of the invention to provide a radial relief for the threadA ridges at the edge of a rolling die, said relief:` being designed to facilitate penetration of the work piece and tominiinize the bending moment arising atthe termination of the thread onthe work piece.

A furtherobject isthe provision of novel thread rolling dies having their` thread starts relievedin` such. af manner that the` dieis capableof producing work pieces conformingy to high tensile bolt standards.

It is also an object ofthe invention to provideA novel threadv rolling diesv which have extended service life and increased resistance to breakage.

Other objects andadvantage's of the novel dies ofl this invention will be apparent fr ornthe. following description thereof and from the accompanying, drawingsfwherein:

Figure l is a sideelevation of arthreadY rolling die made in accordance` with the invention;

Figure 2. is a portionofF the development of: the periphery of the die of Figure 1;

Figure` is a fragmentary sectional, view, onan. enlarged scale, along line` Elf-3` of Figure 2 and; showing-a portion of afnished `work piece` in engagement withgthe-die;`

Figure4 is af fragmentary sectional View, omanenlarged scale, along line `4-4 ofFigureZ;

Figure Sfis another fragmentary sectional view on an enlarged scale, along line ST5 of-y Figure 2 l with, the: abrasive wheel used for grindingl the relief shown ineontact with `one of the thread ridges;

Figure 6 is a, fragmentary view-similaritoFigureZ showing the application of the invention to a rollingdie` haying an Acme thread; andl Figure 7 isan enlargedjfragmentary section-taken along line 7-7 of; Figure 6,. showing the; original and;` relieved thread prole and showingj the` operating surface. ofthe grinding wheel used toproduce. the relief in two? suc* cessive positions onthe rolling die.

The thread rollingzdie lfshown inFigured is generally cylindrical in shape and has a central opening 12 with a keyway 14 for mounting the die uponandfor unitary rotation with a spindle of a thread rolling machine. Thread ridges 16 are formed on the periphery` of the dief 10 and asis well knownin the art, these ridges arelformedf at thehelix" angle of the thread to be rolled on the work` piece. Thisresults in thefcrmation 'ofaj plurality of continuous threads on the die, the number thereof; beingL approximately equal; to the ratio of.- the die pitchV diameterv to the work pitch diameter. In the example shown, the number of these independent'andparallel threads,ispseven,v the, thread on such` a die usually being` referred to as a seven-start threadi It is understood` that the` number off starts may be variedfover` a Wide range asdesired,v

In Figures 1` throughl 5 of the drawing, the crests of the seven thread profiles or starts are -giyen the-refer; ence numbers 1 8, 20, 22, 24, 26, Z8` and 30; Each; start is relievedf near the side 31 of the die by applying.,

3 a grinding wheel to the crest of the thread and gradually reducing Vthe height thereof from a point of tangency 34. When the die has a thread form of the type shown in Figure 3 the crest of each thread is brought to the level of or preferably about .005" below the root line of the remainder of the die. The point 36 is the point on each start at which the start rises from its minimum height. As pointed out below the axial spacing of the point 34 from the adjacent edge of the die is based upon the specifications of the thread to be formed on the work piece. In the example shown, the point 34 is located one and one-half thread pitches axially from the side of the die. The point 36 is located one-half thread pitch from the side surface 32 of the die. From the point 36 to the point 38, at which the thread crest runs olf the side of the die the thread crest is maintained at the same radius from the center 'of the die.

This latter portion of the die, which does not operate on the work piece, is included to add strength to the die and to support the adjacent profile.

The outline of the grinding wheel 40 used to relieve the thread starts of the rolling die of Figure 1 is shown in Figure 5, in contact with the thread start 20 at a point intermediate the point of tangency 34 and a point of minimum height 36. As shown, the grinding wheel 40 is formed with a central groove 42, complementary to the profile of the relief, and with anks 44 and 46, the latter of which is parallel to the flank of the adjacent thread start i8. As the grinding wheel 40 is fed radially toward the axis of the rolling die and axially toward the end of the die, a point is reached at which the flank 46 coincides with the ilank of the adjacent thread start 18 (see Figure 3). This point is the point 36 for the thread start being ground and for the remainder of its axial travel the grinding wheel is not fed radially.

The thread outline used as an example in Figures l through 5 of the drawings has an angle between the thread flanks of 60 and is provided with equal radii at crest and root. As shown in Figures 3, 4, and 5, the profile of the relieved portion of the thread start maintains the original radii while the angle between the thread flanks, formed by the groove 42 in the grinding wheel is substantially increased, being in this case about 120. It will be noted that the retention of the original crest radius 48 has no deleterious eifect upon the penetration characteristics of the die such as would be introduced if the relieving prole were a straight horizontal line. Also, lthe obtuse angle formed between the reliefprofile flank and the original thread prole flank (Figure 4) is acceptable in work pieces' made to high-tensile bolt standards while the sharper angle made by a horizontal truncation of the thread start is not acceptable, due to the serious reduction in tensile strength of the product caused by such sharp corners.

Due to the fact that the radius 50 on the grinding wheel 40 is carried below the root line of the original thread to a point where it is tangent to the extended flank of the adjacent thread, the distance between the center of radius 50 and the center of the crest radius 48 is smaller than the distance between the center of the crest radius 48 and the original root radius 52. Consequently, where the wheel radius 50 intersects the original root radius 52 (as in Figure 5 a corner or ridge 54 is formed which experience has shown must be minimized in order to avoid leaving undesirable markings on the finished work piece. If the angle between the flanks of the groove 42 in the grinding wheel is increased, thus increasing the distance between the centers of radii 48 and 50, Vthe corner 54 is thus reduced. However, for good rolling and to cause the angle between the flank of the relief profile and the original thread flank to be as nearly as possible a straight line, the angle between the Hanks of the groove 42 should approach as nearly as possible the angle between the original flanks of the thread, in this case 60 for best results with a thread form of the type shown in Figure 3. The angle between the flanks of the groove 42 as shown in the drawing represents the best compromise between these conflicting considerations, that is, the angle between the ilank of the relief prole and the original thread ank is made as large as possible consistent with the minimization of the corner 54.

In Figure 4 is shown an interrupted line 56-56, representing the external surface of the work blank and its relation to the rolling die profile. As shown, the work piece has a diameter 58 which corresponds approximately to the pitch diameter of the finished thread and on which the threads are to be rolled. A second and larger diameter 60 forms the body of the blank and is equal to the major diameter of the thread to be rolled. The portion of the blank having the diameter 60 commonly called the grip length is not to be penetrated by the rolling die. Between the diameters 58 and 60, the blank has a transition portion 62 whose form may vary over a wide range and which is here shown as a straight-line tapered portion, the angle oftaper being about 30 on one side. All of the imperfect or p-artial threads formed by the relieved thread starts on the die must lie between the point 64 where the line 62 intersects the major blank diameter and the point 66 where it intersects the root line 67 of the finished thread.

Certain standard specifications applicable to work pieces of the type shown in Figure 4 require that the thread run-out on the transition portion 62 terminate approximately 1A of one thread pitch away from the point 64 and that the axial distance from the run-out to the point 34 where the crest is at full height be a minimum of one thread pitch and a maximum of two thread pitches.

These requirements dictate that the point 36 contact the work blank approximately 1A pitch from the point 64 and that the point 34 be at least one pitch from the point 36. It has been discovered that the minimum spacing of one thread pitch best avoids the possibility of terminating the run-out at a point objectionably close to the point 64 which would necessitate scrapping the relatively expensive work pieces.

Figures 6 and 7 illustrate the application of a relief profile of the same general type to an Acme or worm thread. 'Ihe original thread prole shown as an example in the drawings is that of a worm thread having a 20 pressure angle; that is, the angle between thread flanks is 40. Since in this case each profile ridge 70, 72 etc.- on the die 74 has a at crest 76, greater pressure is applied to the work piece than in the case of la profile with a sharper crest. At the side edge 78 of the die 74 this pressure becomes a bending force tending to deform the axis of the Work piece. This condition is alleviated by applying a radial relief to each thread start at the side of the die in the manner previously described herein.

As shown in Figure 7, a grinding wheel 80, inclined at the helix angle of the thread on the die and having an annular groove formed by the flanks 82 in its periphery, is applied to each individual thread ridge. The Wheel 80 is shown at the middle of Figure 7 at its point of initial contact with the thread ridge 72 and this point should be the same axial distance from the side edge of the die for every thread ridge. From this position, as the grinding operation proceeds, the axis of the grinding wheel is caused to approach the axis of the die in a regular manner, following the helix of the thread until the grinding wheel runs off the side of the die is shown at the left side of Figure 7.

It will be seen that thus the flat crest of the ridge 72 is eliminated and the ridge gradually takes the formA of the groove in the grinding wheel 80, comprising the anks 82 with a small radius 84 joining the flanks. Therefore the terminal portion of the thread rolling ridge on thedie has the radius 86 equal to radius 84 at the crest thereof, substantially reducing the bending force acting on the work piece and facilitating penetration.

The 4angle between the Hanks 82 vis made suchas to afford optimum penetration characteristics which are dependent upon the material of the Work piece. In this case the wide root 88 of the thread profile on t-he die affords more latitude with respect to the angle between the flanks 82 and no objectionable corners such as the corner at 54 discussed above are formed so long as the width of the groove in the grinding wheel is made substantially equal to the Width of the thread ridge at its root.

Also, in most cases, it is not necessary to carry the relief radially inward of the die of Figure 7 to a point at or below the level of the root of the thread starts as was done in the case of the die of Figure l because of the different standards which apply to Work pieces having Acme threads.

The invention may be embodied in 'other specic forms without departure from the spirit of essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

l. A thread rolling die of substantially cylindrical form having a plurality of thread forming ridges thereon, each ridge starting at one side of the die and extending helically around the cylindrical surface of the die to the other side thereof, each thread forming ridge having a prole constituted by a crest and side flanks, the start of cach thread forming ridge being relieved so that a line joining successive points on the apex of the crest of the start will generate a curve having its origin substantially at the root line at the side of the die and its terminus at a point of tangency with the full height of the thread ridge, the successive profiles of each start from the beginning to the full height thereof being uniform and symmetrical about a perpendicular to the crest and the crests of successive profiles being similar to the crest of the ful-l height section of the thread forming ridge, whereby the root formed by the thread starts and the root formed by `the full height thread forming ridges of the die will be substantially idental in shape and width.

2. The thread rolling die according to claim l wherein the crest of the relieved portion of the thread ridge is formed on a radius substantially equal -to the radius of the crest of a thread ridge of full height.

3. The thread rolling die according to claim 1 wherein the angle between the flanks of the relieved portion of the thread ridge is substantially less than but substantially greater than the angle included between the anks of a thread ridge of full height.

References Cited in the le of this patent UNITED STATES PATENTS 77,710 Beecher May 12, 1868 446,932 Simonds Feb. 24, 1891 1,946,735 Frayer Feb. 13, 1934 OTHER REFERENCES Page 5 of Namco Ifhread and Form Rolling Machine, Bulletin No. TR-47 by the National Acme Co., Cleveland 8, Ohio.

Page 48 of Thread Rolling Handbook, Revised Edition Catalogue No. 10 by Roll Thread Die Company, Wer,u meuse,- 

